Method for varying x-ray hybrid resist space dimensions

ABSTRACT

The present invention provides combining the advantages of hybrid resist with the unique properties of x-ray lithography to form high tolerance devices with x-ray pitch and to provide a means for varying the space width and fine tuning to account for process variations. Accordingly, a space width in the hybrid resist can be selectively printed by varying the mask-wafer gap distance, allowing more versatile structures to be formed and adjustments to be made for process changes such as resist composition and ion implant levels.

RELATED APPLICATIONS

This application is related to the following U.S. Patent applications:"Method of Photolithographically Defining Three Regions with One MaskStep and Self-Aligned Isolation Structure Formed Thereby," Ser. No.08/895,748, filed Jul. 17, 1997, "Method for Forming Sidewall SpacersUsing Frequency Doubling Hybrid Resist and Device Formed Thereby," Ser.No. 08/895,749, filed Jul. 17, 1997, "Low `K` Factor HybridPhotoresist," Ser. No. 08/715,288, filed Sep. 16, 1996, and "FrequencyDoubling Photoresist," Ser. No. 08/715,287, filed Sep. 16, 1996.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to the field of semi-conductormanufacturing and, more specifically, to a method for optimizing the useof x-ray lithography in conjunction with hybrid resists.

2. Background Art

The need to remain cost and performance competitive in the production ofsemiconductor devices has caused continually increasing device densityin integrated circuits. To facilitate the increase in device density,new technologies are constantly needed to allow the feature size ofthese semiconductor devices to be reduced. For the past 20 years,optical lithography has driven the device density and the industry hasresorted to optical enhancements to allow increasing densities. As anexample, some such enhancements include overexposing/overdeveloping,hard and soft phase shifts, phase edge masks, and edge shadowing.Unfortunately, the latest of such enhancements tend to offer only minorincreases in density and the limit of optical enhancements appearsinevitable in the near future.

The same industry trend to increase device density is also causing atransition to x-ray lithography. X-rays are a desirable type of exposureradiation because the wavelength of x-rays (about 8 Å) is smaller thanthe wavelength of ultra-violet radiation typically used to fabricatedense integrated circuits. The smaller wavelength allows for exposure ofa resist through a mask having a smaller image than in opticallithography. However, certain aspects of x-ray lithography areinsufficiently advanced to satisfy the current demand for increasingdevice density.

In optical lithography, projection printing allows the mask image to beprojected onto the resist at a reduced size to increase the possibledevice density. For example, a 4× reduction is typical. However,sufficient success has not been achieved in attempts to fabricate lensesfor reducing the size of the x-ray mask image to a smaller aerial image.Accordingly, even though the potential exists for exposing a 0.05 μmaerial image onto the resist using x-ray lithography, it requires a maskhaving a 0.05 μm image. Producing such a mask within required tolerancescan be a formidable task and constitutes the most significantdisadvantage of x-ray lithography. In fact, the tolerances that areachieved in the x-ray mask dictate the tolerances that can be achievedin a product produced using x-ray lithography.

The smaller images on a x-ray mask are more difficult to fabricate thanthe larger images on an optical mask because of the smaller image sizeand different materials. The smaller size requires electron beamlithography (EBL) to carve out the image and EBL has not yet advancedsufficiently to produce masks that take full advantage of the smallerwavelength. Further, instead of a chrome mask layer used in opticallithography, the nature of x-rays requires tungsten, gold, or othermaterial with a high x-ray extinction coefficient that must also be muchthicker than the typical chrome layer. Unfortunately, the high x-rayextinction materials are difficult to control within tolerance and thethickness increases the difficulty of mask fabrication.

Also, x-ray lithography involves proximity printing the mask image ontothe resist. Proximity printing simply means that the mask is in closeproximity to, but not in contact with, the surface of the resist layer.The gap distance between the mask and the wafer is minimized to producean aerial image through the mask with as high a contrast possible. Thatis, the gap distance is decreased so that the transition from zerointensity to full intensity occurs over a smaller area. Typical gapdistances are between 10 and 50 μm.

The pitch, or combined width of an adjoining line and space in asemiconductor, can theoretically be very small when x-ray lithography isused, but the poor tolerance of the x-ray mask prevents the preciseformation of reliable devices at x-ray pitch. Even though thepossibility of x-ray pitch exists, abnormalities in the x-ray mask willyield abnormalities in lines and spaces sufficient to precludefabricating reliable devices as small as allowed by the small x-raywavelength. It would be an improvement in the art to provide a methodfor forming high tolerance devices with x-ray pitch. Such a method mustyield few enough abnormalities in lines and spaces to provideperformance of the final product within industry standards. Without amethod for forming high tolerance devices at x-ray pitch, the value ofx-ray lithography for increasing device density is seriously diminishedand advancement in improving chip cost and performance may stagnate.

DISCLOSURE OF INVENTION

Accordingly, the present invention provides a method for defining highdensity features on semiconductor devices. The novel method uses hybridresist and x-ray lithography to define these features. The method avoidsthe problems in accurately forming x-ray masks at the feature sizedimensions by using the unique properties of hybrid resist to formspaces in the resist where an intermediate exposure occurs, in otherwords, to use an edge printing technique. These spaces have a dimensionthat is independent of the feature size of the mask and is smaller thanthe typical feature size of a conventional x-ray mask. Thus, the presentinvention is able to consistently form small, high tolerance featuresusing x-ray lithography without requiring equally small and hightolerance mask shapes in the x-ray mask. The present invention methoduses adjustments in the mask-wafer gap distance during x-ray exposure toform spaces in hybrid resist of a desirable dimension. In particular,the size of a space formed in hybrid resist is determined by the aerialimage which, in turn, is varied by adjusting the gap distance duringexposure. This can be used to provide accurate patterning of hybridresist with different space widths. Additionally, this method can beused to compensate for process variations that would otherwise causeunwanted changes in space widths.

Within a range of about 10-50 μm as the mask-wafer gap distance, one cantake advantage of the combined properties of x-ray and hybrid resist toincrease the space width in the resist by increasing the gap distancebetween the mask and wafer. In this sense, the gap distance is not theproblem, but rather the solution to the need for varying the hybridresist space width and fine tuning to account for process variations. Inaddition, a x-ray hybrid resist yields high tolerance lines and spaceslargely independent of a low tolerance x-ray mask. Only the transitionfrom exposed resist to unexposed resist at the edge of the x-ray aerialimage is involved in producing a corresponding space. The width of thespace produced is generally not dependent on the size of the mask image.Thus, an x-ray mask image wherein the reticle opening is either toosmall or too large and causes a fatal defect in a typical resist mightnot yield such an effect in a hybrid resist. Accordingly, a space widthin the hybrid resist can be selectively printed by varying themask-wafer gap, allowing more versatile structures to be formed andadjustments to be made for process variations such as resist compositionand ion implant levels.

It is an advantage of the present invention that space width can bevaried in the production of high tolerance spaces at x-ray pitch.

It is an additional advantage that adjustments can be made in spacedimensions at x-ray pitch to compensate for process variations.

The foregoing and other advantages and features of the invention will beapparent from the following more particular description of a preferredembodiment of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

The preferred embodiments of the present invention will hereinafter bedescribed in conjunction with the appended drawings, wherein likedesignations denote like elements, and:

FIG. 1 is a schematic diagram showing the use of the hybrid resist;

FIG. 2 is a graph is illustrating how positive resist undergoes anincrease in solubility as the exposure dose is increased;

FIG. 3 is a graph illustrating the line pattern for positive resistprinted with a reticle line pattern;

FIG. 4 is a graph illustrating how in negative resist systems exposedareas undergo a reduction in solubility as the exposure dose isincreased;

FIG. 5 is a graph illustrating the line pattern for negative resistprinted with a reticle line pattern;

FIG. 6 is a graph of the resist solubility as a function of exposuredose for hybrid resist;

FIG. 7 is a graph illustrating the space/line/space pattern formed ontoa substrate using hybrid resist; and

FIG. 8 is a graph of linewidth in nanometers (nm) plotted against focusin microns (μm) of a formulation of a standard negative resist atvarious exposure energies;

FIG. 9 is a graph of linewidth for a negative tone line of a hybridpattern in nm plotted against focus in μm of a hybrid resist of thepresent invention at various exposure energies;

FIG. 10 is a graph showing the linewidth in nm plotted against theamount of positive tone solubility inhibitor (MOP) incorporated in ahybrid resist of the present invention;

FIG. 11 is a comparative model of what the range of focus is for a givenlinewidth using standard resist formulations and a hybrid resistformulation of the present invention;

FIG. 12 is a graph showing the dissolution rate in nanometers per second(nm/sec) as a function of the exposure dose in millijoules (mJ) usingone formulation of a hybrid resist of the present invention;

FIG. 13 is a graph showing the resultant line width and constant spacewidth as functions of the chrome space width using one formulation of ahybrid resist of the present invention;

FIG. 14 is a graph showing the dissolution rate of an alternativeformulation of the hybrid resist in nm/sec as a function of the exposuredose in mJ;

FIG. 15 is a graph showing the variation in space width in μm as afunction of MOP loading using one formulation of hybrid resist of thepresent invention;

FIG. 16 is a graph of the response of a formulation of the hybrid resistof the present invention in which exposed (negative) line, unexposed(positive) line and space widths are plotted as a function of exposuredose;

FIG. 17 is a schematic view of an exemplary mask portion;

FIG. 18 is a top schematic view of a wafer portion with patterned hybridresist upon it;

FIG. 19 is a cross-sectional side view of the wafer portion of FIG. 18taken along line 19--19;

FIG. 20 is a cross-sectional side view of the wafer portion of FIG. 18taken along line 20--20;

FIG. 21 is a top schematic view of a wafer portion with patterned hybridresist and positive tone portions removed;

FIG. 22 is a cross-sectional side view of the wafer portion of FIG. 21taken along line 22--22;

FIG. 23 is a cross-sectional side view of the wafer portion of FIG. 21taken along line 23--23; and

FIG. 24 is a graph of the energy intensity produced by an x-ray maskimage of 150 nm equal lines and spaces at a 25 μm gap;

FIG. 25 is a graph of the energy intensity produced by an x-ray maskimage of 150 nm equal lines and spaces at a 35 μm gap;

FIG. 26 is a graph superimposing the energy intensity curve of FIG. 24onto the curve of FIG. 25;

FIG. 27 is a diagram showing how the x-ray energy intensity curve at a25 μm gap determines the width of spaces formed in a hybrid resist;

FIG. 28 is diagram showing how the x-ray energy intensity curve at a 35μm gap determines an increased width of spaces formed in a hybridresist; and

FIG. 29 is a flowchart showing a process for forming a hybrid resistspace using x-ray lithography according to a preferred embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiment of the present invention overcomes thelimitations of current technology by combining the advantages of hybridresist with the unique properties of x-ray lithography. The presentinvention provides a method to produce high tolerance devices with x-raypitch (the combined width of an adjoining line and space achieved usingx-ray lithography) and a means to vary the width of spaces produced in ahybrid resist and to fine tune to account for process variations.Normally, the aerial image sharpness is a problem in x-ray lithographyand the smallest possible mask-wafer gap distance is desired to yield animage with the highest contrast. The present invention method usesadjustments in the gap distance during x-ray exposure to form spaces inhybrid resist of a desirable dimension. In particular, the size of aspace formed in hybrid resist is determined by adjusting the gapdistance during exposure. This can be used to provide accuratepatterning of hybrid resist with different space widths. Additionally,this method can be used to compensate for process variations that wouldotherwise cause unwanted changes in space widths. A description ofhybrid resist will now be given, followed by a description of thepreferred embodiments.

Hybrid Photoresist

The preferred embodiment uses photoresist material having,simultaneously, both a positive tone and a negative tone response toexposure. This combination of materials can provide a new type ofresist, which we call a hybrid resist.

As a hybrid resist is exposed with actinic radiation, areas exposed withhigh intensity radiation form a negative tone line pattern. Areas whichare unexposed remain insoluble in developer, thus forming a positivetone line pattern. Areas which are exposed with intermediate amounts ofintensity, such as the edges of the aerial image where diffractioneffects have reduced the intensity, form a space in the resist filmduring develop. This resist response is an expression of the uniquedissolution rate properties of this resist, in which unexposed resistdoes not develop, partially exposed resist develops at a high rate, andhighly exposed resist does not develop.

The unique dissolution rate response of the hybrid photoresist allows asingle aerial image to be printed as a space/line/space combinationrather than as a single line or space, as with conventional resist. This`frequency doubling` capability of this resist allows conventionalexpose systems to be extended to higher pattern densities. It is anadvantage of one example of hybrid resist that lines and spaces of 0.2μm and less can be printed with current deep ultra violet (DUV)lithography tools that are designed for operation at 0.35 μm resolution.

It is a further advantage of this type of hybrid resist that the spacewidth is generally unchanging as the exposure dose and the reticle imagesize are changed. This allows very precise image control for the spacewidth within each chip, across each wafer, and from one batch of productwafers to the next.

Still another advantage the hybrid resist is the relaxation of theminimum reticle feature size due to the frequency doubling capability ofhybrid resist. For example, to print a 0.2 μm feature with conventionalresist generally requires a 0.2 μm reticle image size. With hybridresist, a 0.2 μm space can be formed with a single edge of a reticlefeature; for example, a 0.5 μm reticle opening could produce two 0.2 μmspaces and a 0.2 μm line. In this way, one could accomplish `reduction`x-ray or E-beam lithography; the reticle image pitch could beapproximately 2× the printed pitch on the substrate. This also has theadditional advantage of allowing a relaxation of the image sizerequirements of optical reticles, reducing cost and improving yield ofthe reticle. It is an advantage of hybrid resist that lines and spacesof 0.2 μm and less may be achieved without altering the present opticallithography tools.

It is a further advantage that the space width is generally unchangingas the exposure dose and reticle sizes change, thereby allowing greaterprocess latitude for control of space width. Through the use of thehybrid resist of the present invention, errors in the image dimension onthe reticle are not reproduced in the space width printed on thesubstrate. As a result, the across-chip space width variation isminimized. This is valuable for optical, X-ray and e-beam exposuremethods. It is especially useful in lithographic techniques that requirea 1× reticle, i.e., a reticle that normally has a one-to-onerelationship with the image printed on the substrate, because variationsin the image size on the reticle are normally reproduced on thesubstrate.

Accordingly, the preferred embodiment hybrid resist provides aphotoresist material having, simultaneously, both a positive tone and anegative tone response to exposure. The positive tone response dominatesat the lower exposure dose while the negative response predominates atthe higher exposure dosages. Exposure of this resist creates aspace/line/space combination, whereas either of the conventional resistswould produce only a single feature. Turning to FIG. 2, a graph isillustrated showing how positive resist undergoes an increase insolubility as the exposure dose is increased. Turning to FIG. 3, theline pattern for positive resist printed with a reticle line pattern isillustrated.

On the other hand, in the negative resist system exposed areas undergo areduction in solubility as the exposure dose is increased, asillustrated in FIG. 4. Turning to FIG. 5, the line pattern for negativeresist printed with a reticle line pattern is illustrated.

For the hybrid resist of the present invention, the positive toneresponse causes an increase in solubility in the areas where diffractioneffects have reduced the exposure intensity, such as the areas near theedge of the reticle image. As the exposure dose is increased, thenegative tone response predominates, causing a reduction in solubilityin the more highly exposed areas. Turning to FIG. 6, the graph of theresist solubility as a function of exposure dose for hybrid resist isillustrated. Printing a reticle line pattern onto a substrate results inthe space/line/space pattern illustrated in FIG. 7.

In this manner, the aerial image is "frequency doubled" to produce twicethe number of features than would otherwise be attainable with thestandard resist. FIG. 1 illustrates these salient differences between apositive resist, a negative resist, and a hybrid resist. In FIG. 1, aphotoresist 140 has been deposited over the surface of substrate 150. Amask 120 with chrome areas 130 is used to selectively mask portions ofphotoresist 140 from an optical radiation source. After exposure,photoresist 140 is developed and portions subsequently removed bywashing the surface of the wafer. Depending on the nature andcomposition of photoresist 140, a certain pattern, which is related tochrome areas 130 on mask 120, will be formed in photoresist 140. Asshown in FIG. 1, a positive photoresist will leave areas that correspondto chrome areas 130. A negative photoresist will create a patternwhereby the areas that correspond to chrome areas 130 are removed fromsubstrate 150. A hybrid photoresist material will leave a photoresistpattern that corresponds to removal of the photoresist material from theareas of substrate 150 that are associated with the edges of chromeareas 130.

The frequency doubling hybrid resist is typically formulated usingcomponents of existing positive and negative tone resists. Thisincludes, for example, poly(hydroxystyrene) resins which are partiallymodified with acid-sensitive solubility dissolution inhibitingfunctionalities, a cross-linker, a photo-acid generator, and,optionally, a base additive and a photosensitizer.

The resist formulations may be varied to obtain a fast positive tonereaction and a slow negative tone reaction for optimal results.Additionally, the positive tone component can be chosen such that it isrelatively insensitive to post expose bake temperatures, while thenegative tone portion is chosen to be more highly sensitive to postexpose bake temperatures. In this way, the relative sensitivity of thepositive and negative responses can be altered with bake temperatures toprovide the desired imaging results.

In addition, the resist formulations may be altered to provide spacewidths of different dimensions. For example, as the amount of solubilityinhibitor on the poly(hydroxystyrene) resin is increased, the printedspace width becomes smaller (FIG. 15). This approach may also be used toalter the isofocal print bias of the negative tone line; at higherpositive tone solubility inhibitor concentrations, the isofocal printbias of the negative tone line increases (FIG. 10). This is desirable insome applications for reducing the size of the printed negative toneline, optimizing the frequency doubling characteristics of the resist.

The relative responses of the positive and negative tone functions ofthe hybrid resist can also be altered by modifying the exposureconditions. For example, the negative tone line of the hybrid resistdoes vary with exposure dose and reticle dimension, similar to thebehavior of a conventional resist. Thus, as exposure dose is increased,for example, the negative tone line increases in width, and the spacesremain the same size, but the spaces are shifted to a new position onthe substrate, since they lie adjacent to the negative line. Similarly,the positive tone lines alter in size as the exposure dose or reticledimension are altered.

As another example, two reticles could be used to print two separatepatterns in the resist. One reticle could be exposed with a high dose,causing the hybrid functions to be expressed in the resist. Anotherreticle could be exposed in the same resist film at a lower dose,causing only the positive tone function to be expressed in that portionof the resist. This effect could also be accomplished with a singleexpose process if, for example, the reticle contained a partial filterof the actinic radiation in the areas where a lower exposure dose wasdesired. This allows wider spaces to be printed at the same time as thenarrower features, which is necessary in some device applications.

In a modification of this two-step imaging approach, a hybrid resist canbe used to create a standard negative tone pattern. If the resist filmis image-wise exposed with a standard negative tone reticle, baked toform the hybrid image, then blanket exposed with actinic radiation anddeveloped without a second post-expose bake process, the result is astandard negative tone image. This approach may be desirable in someapplications, such as the formation of gate conductor circuits, whichrequire very small lines to be printed, but do not require a highdensity image pitch. As an alternative to this method, the resist may beblanket exposed to a low dose of actinic energy after the image-wiseexposure and before the baking step. The desirability of the methodwould depend on whether a solubility inhibiting protective group ispresent on the resin and whether the positive tone response istemperature dependent.

An advantage of using the hybrid resist in such applications is that thenegative tone line of the hybrid resist can exhibit a large print biasat its isofocal point, as shown in FIG. 9. In other words, at the pointof largest process latitude for the hybrid negative tone line, theresist image size can be substantially smaller than the reticle imagesize. This is desirable because the aerial image is less degraded bydiffraction effects at the larger reticle size, thus allowing a largerdepth of focus in optical lithography to be attained than is possiblewith conventional positive and negative tone systems, as shown in FIG.8. This print bias is a result of the fact that the edge of the chromeline prints as a space. The space, in effect, acts to `trim` the edgesof the aerial image, causing the negative line to print smaller than itwould with a conventional negative resist. This is an expression of thefrequency doubling character of a hybrid resist.

It is possible to design the resist formulation to optimize the printbias of the negative tone line. For example, by choosing an appropriateloading factor for the positive tone solubility inhibitor, one mayobtain a particular print bias as shown in FIG. 10. In theory, it isquite obvious that similar variations in the photoresist response canalso be brought about by making appropriate changes in concentrationsand reactivities of other components as well.

For example, we have found that with exposure on a DUV 0.5 NAlithography tool, the isofocal print bias for a hybrid resist can be0.11 μm larger than the isofocal print bias for a standard negative toneresist, as exemplified in FIGS. 8 and 9 when standard calculations knownin the art are performed on the data. This difference can be utilized intwo ways. In one approach, the same reticle image size could be usedwith the hybrid resist to print a smaller line than the standard resist,while maintaining focus and exposure process latitude. In another mannerof use, the size of the reticle features could be increased with thehybrid resist relative to the standard resist, while printing the sameimage size as the standard resist. The use of a larger reticle imageprovides a larger depth of focus due to reduced diffraction effects, asshown in the graph of FIG. 11. In the former application, higherperformance is achieved with the smaller size of the hybrid resist. Inthe latter application, higher yield is achieved due to the largerprocess latitude of the hybrid resist.

The resist formulations may be varied to obtain a high photospeedpositive tone reaction and a low photospeed negative tone reaction foroptimal results. Additionally, the positive tone resist may be chosen sothat it is insensitive to post expose bake (PEB) conditions so that theratio of sensitivity of the positive tone to the negative tone functioncan be altered, thus changing the ratios of the space/line/spacecombinations.

Another option for changing the space/line/space ratios is to utilize agray-scale filter in the reticle of the exposure tool. A gray scalefilter only allows a portion of the radiation to pass through thereticle, thereby creating areas of intermediate exposure. This preventsthe negative tone resist function from operating in these areas becausethe exposure dose would never reach the critical point, but would stillallow the positive functions to occur, thereby creating wider spaces.This allows wider spaces to be printed at the same time as the narrowerfeatures, which is necessary in some device applications.

The following examples are exemplary of the frequency doubling resistcomposition, but are not meant to be limiting and many variations willbe readily apparent to one of ordinary skill in the art.

The photoresist resins suitable for use in accordance with the inventioninclude any of the base-soluble, long chain polymers suitable for use asa polymer resin in a photoresist formulation. Specific examples include:(i) aromatic polymers having an --OH group, e.g., polyhydroxystyrenessuch as poly (4-hydroxystyrene), poly (3-hydroxystyrene), commerciallyavailable from Hoechst Celanese of Corpus Christi; Tex., novolak resinscommercially available from Shipley of Marlboro, Mass.; and polymershaving a phenolic --OH group, e.g., phenol formaldehyde resins; (ii)polymers having an acid group, e.g., polymethacrylic acid with an esterside chain; and (iii) acrylamide group type polymers.

The polymer resin in its deprotected form (i.e., once the positive tonereaction has occurred) is base soluble and compatible with developersolutions, such as aqueous solutions of metal-free ammonium hydroxide,tetramethylammonium hydroxide, and tetraethyl ammonium hydroxide, metalcontaining potassium hydroxide, and sodium metasilicate. Preferredpolymer resins have an average molecular weight within the range ofabout 1,000 daltons to about 250,000 daltons, and most preferably withinthe range of about 1,000 to 25,000 daltons, to enhance its solubility indeveloper solutions. Examples include p-hydroxystyrene-maleic acidanhydiride copolymers,polyhydroxystyrene-p-tertiarybutyl-carganatostyrene co-polymers,poly(2-hydroxystyrene), phenol-formaldehyde resins, polymethylmethacrylate- tertiary butyl methacrylate-polymethacrylic acidterpolymers, poly-4-hydroxystyrene-tertiary butyl methacrylatecopolymers, poly(4-hydroxystyrene) with one or more acid labile alkyl oraryl substituents on the aromatic ring, a poly(3-hydroxystyrene) withone or more allyl or aryl substituents on the aromatic ring, or any ofthese as the major number of subunits in a copolymer, such as PHM-C,commercially available from Maruzen America of New York, N.Y. The PHM-Cincludes both poly (hydroxystyrene) subunits and vinyl cyclohexanolsubunits preferably being in the range of about 99:1 to about 50:50. Themost preferred ratio is about 90 poly (hydroxystyrene) units to about 10vinyl cyclohexanol subunits.

Crosslinking compositions are preferably tetramethoxymethyl glycouril("Powderlink") and 2,6-bis(hydroxymethyl)-p-cresol. However, otherpossible crosslinking compositions include: ##STR1## their analogs andderivatives, as can be found in Japanese Laid-Open Patent Application(Kokai) No. 1-293339, as well as etherified amino resins, for examplemethylated or butylated melamine resins (N-methoxymethyl- orN-butoxymethyl-melamine respectively) or methylated/butylatedglycol-urils, for example of the formula: ##STR2## as can be found inCanadian Patent No. 1 204 547.

Photoacid generators ("PAG") include, but are not limited to:N-(trifluoromethylsulfonyloxy)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboximide("MDT"), onium salts, aromatic diazonium salts, sulfonium salts,diaryliodonium salts and sulfonic acid esters of N-hydroxyamides or-imides, as disclosed in U.S. Pat. No. 4,731,605, incorporated herein byreference. Also, a PAG that produces a weaker acid such as dodecanesulfonate of N-hydroxy-naphthalimide ("DDSN") may be used.

Possible base additives include, but are not limited to: dimethylaminopyridine, 7-diethylamino-4-methyl coumarin ("Coumarin I"), tertiaryamines, proton sponge, berberine, and the polymeric amines as in the"Pluronic" or "Tetronic" series from BASF. Additionally, tetra alkylammonium hydroxides or cetyltrimethyl ammonium hydroxide, may be usedwhen the PAG is an onium salt.

Examples of sensitizers that may be utilized include: chrysenes,pyrenes, fluoranthenes, anthrones, benzophenones, thioxanthones, andanthracenes, such as 9-anthracene methanol (9-AM). Additional anthracenederivative sensitizers are disclosed in U.S. Pat. No. 4,371,605, whichis incorporated herein by reference. The sensitizer may include oxygenor sulfur. The preferred sensitizers will be nitrogen free, because thepresence of nitrogen, e.g., an amine or phenothiazine group, tends tosequester the free acid generated during the exposure process and theformulation will lose photosensitivity.

A casting solvent is used to provide proper consistency to the entirecomposition so that it may be applied to the substrate surface withoutthe layer being too thick or too thin. Sample casting solvents include:ethoxyethylpropionate ("EEP"), a combination of EEP and γ-butyrolactone("GBL"), and propyleneglycolmonoethylether acetate (PM acetate).

In the following Examples, one of each of these has been chosen,however, it is to be recognized that many other compositions may beselected for various portions of the resist. In the broadest sense, themethod and structure of the preferred embodiment may be achieved usingany hybrid resist comprised of a negative tone component and a positivetone component, wherein the positive tone component acts at a firstactinic energy level and the negative tone component acts at a secondactinic energy level, the first and second actinic energy levels beingseparated by an intermediate range of actinic energy levels.

EXAMPLE 1

The following compositions were dissolved in propylene-glycolmonomethylether acetate (PM acetate) solvent available from Pacific Pac,Inc., Hollister, Calif. containing 350 ppm of FC430, a non-ionicfluorinated alkyl ester surfactant available from 3M, St. Paul, Minn.for a total of 20% solids:

poly(hydroxystyrene) (PHS), 10% hydrogenated, available from MaruzenAmerica, New York, N.Y. with about 25% of the phenol groups protectedwith methoxypropene (MOP), 81.2% of solids;

N-(trifIuoromethylsulfonyloxy)-bicyclo-[2.2.1]-hept-5-ene-2,3-dicarboximide(MDT), available from Daychem Labs, Centerville, Ohio, 10.5% of solids;

tetramethoxymethyl glycouril (Powderlink), available from Cytec,Danbury, Conn., 8.2% of solids; and

7-diethylamino-4-methyl coumarin dye (Coumarin 1), available from theAldrich Chemical Company, 0.1% of solids.

The solution was filtered through a 0.2 μm filter. The solution wascoated onto silicon wafers primed with hexamethyl-disilaaane with a softbake of 110° Celsius (C) resulting in films of about 0.8 μm thick asdetermined by a Nanospec reflectance spectrophotometer. The coatedwafers were then exposed with deep ultraviolet (DUV) excimer laserradiation having a wavelength of 248 nm in a 0.37 numerical aperture(NA) Canon stepper with a matrix of different doses from low doses tohigh doses and post expose baked (PEB) at 110° C. for 90 sec. Thedissolution rates of the exposed films were calculated from thethickness of remaining film after developing for a given amount of timewith 0.14 Normal (N) tetramethylammonium hydroxide (TMAH) developer. Thedissolution rate vs. exposure dose relationship is shown in FIG. 6. Asshown in FIG. 6, the resist has a very low dissolution rate (about 2nm/sec) when unexposed. As the dose is increased, the dissolution rateincreases until reaching about 50 nm/sec. The dissolution rate remainsrelatively constant at this level in the dose range of about 1millijoule (mJ) to about 3 mJ. Increasing the dose further, the negativecross-linking chemistry becomes predominant and the dissolution ratefalls back to a value close to zero.

In another experiment with the same resist, when a MICRASCAN II 0.5NADUV stepper is used to expose an isolated chrome space onto the hybridresist film, the space/line/space measurements as a function of width ofthe chrome space are plotted, as shown in FIG. 13. The data suggeststhat, although the width of the line increases correspondingly with thatof the chrome space on the mask, the space on either side of the lineremains relatively constant.

EXAMPLE 2

This example illustrates the manner in which changing the type ofphotoacid generator and relative amounts of the various components canchange the dissolution rate characteristics of the hybrid resist andsubsequently the lithographic response. This second formulation wasprepared and processed in a manner similar to EXAMPLE 1, however, it iscomprised of the following components:

PHS with about 25% of the phenol groups protected with MOP, 90.8% ofsolids;

triphenyl sulfonium triflate, 1.3% of solids;

Powderlink, 7.8% of solids;

tetrabutyl ammonium hydroxide base, 0.1% of solids; and

sufficient PM acetate containing 350 ppm FC-430 surfactant as a solventto form a 18.9% solids solution.

The dissolution rate characteristic of the resulting hybrid resist isshown in FIG. 14. The overall nature of the curve remains similar tothat shown by the hybrid resist of EXAMPLE 1, in that the dissolutionrate starts out low for an unexposed resist, increases to a high atabout 5 mJ and decreases to a low above 7 mJ. However, the absolute doserange and the dissolution rates within these ranges are quite differentfrom those shown in FIG. 12.

FIG. 16 represents the response of this formulation of the hybrid resistwhen exposed through a mask of nested chrome lines and spaces of equalwidths in a MICRASCAN II DUV 0.5 NA stepper tool. Negative line,unexposed (positive) line and space widths are plotted as a function ofexposure dose. The space remains relatively constant in the range ofabout 0.18 μm, whereas both lines vary as the exposure dose is varied.

EXAMPLE 3

This example illustrates that the space width of the frequency doubledimage can be changed by varying the protection level of PHS with MOP.Two different PHS lots having 24% and 15% MOP loading, respectively,were used to make hybrid formulations identical to that of EXAMPLE 1,except that the total solids contents were adjusted to 16.0% of thetotal to obtain film thicknesses of about 0.5 μm. From these two stockformulations, several other formulations with average MOP levels rangingfrom 15 to 24% were prepared. Wafers were coated and soft baked at 110°C., exposed on a MICRASCAN II DUV 0.5 NA stepper, post exposed baked at110° C. for 60 sec and finally developed with 0.14N TMAH developer. Areticle with an isolated chrome opening was printed in a hybrid resistfilm. The spacewidth of the resist image was measured and graphed as afunction of the average MOP solubility inhibitor loading in the PHS usedfor making the respective formulations. It was found that the spacewidth was strongly dependent on MOP concentration, as shown in FIG. 15.

EXAMPLE 4

Negative tone imaging may be performed with the hybrid resist of thepresent invention, using a blanket DUV expose after the PEB and prior tothe develop.

A hybrid resist formulation as described in EXAMPLE 2, above, wasimage-wise exposed with a chrome reticle with an electrical test patternon a 0.5NA DUV expose system. Silicon wafers (200 mm) with a 2000Angstrom (Å) film of polysilicon were used as a substrate so that theresulting etched patterns of the resist image could be measured withelectrical probe techniques. After the post expose bake process, thewafers were cycled back into the expose tool (MICRASCAN II) and exposedat 10 mJ per square centimeter (cm²) with a clear glass reticle. A postexpose bake process was not performed after the second exposure. Thepurpose of the second exposure was to remove the initially unexposedresist from the wafer, leaving only a negative tone resist pattern afterdevelop.

The initial image-wise expose dose was 17-24 mJ/cm2, the post exposebake temperature was 110° C. for 90 sec and the develop time was 100 secin 0.14N TMAH. A standard negative tone resist was processed in asimilar fashion, with the omission of a blanket expose step as acontrol. The electrical data from this experiment is shown in FIGS. 8and 9. A large isofocal print bias of approximately 0.11 μm was observedfor the hybrid resist relative to the standard negative resist, ascalculated using standard methods known in the art.

When hybrid resist is exposed to actinic energy, areas of the resistwhich are subject to a fall exposure form a negative tone line pattern,areas which are unexposed form a positive tone pattern, and areas whichare exposed to intermediate amounts of radiation become soluble and washaway during development. Turning to FIG. 17, an exemplary mask 1700containing a mask blocking shape 1702 is illustrated. When hybrid resistis deposited on a wafer, exposed through mask 1700 with actinicradiation, and developed, the mask 1700 creates the a "linked" or"donut" pattern in the hybrid resist. Such a linked pattern isillustrated FIGS. 18, 19 and 20, where FIG. 19 is a cross section of thewafer in FIG. 18 taken along lines 19--19, and FIG. 20 is a crosssection of the wafer in FIG. 18 taken along lines 20--20.

FIG. 18 shows a wafer portion 1802 upon which hybrid resist has beendeposited, exposed through mask 1700 containing a blocking shape 1702and developed. Hybrid resist portions which are unexposed (ie., theinside region 1804 blocked by mask shape 1702) remain photoactive andinsoluble in the developer and form positive tone line patterns. Hybridresist portions which are exposed with high intensity radiation (i.e.,the outside region 1806 not blocked by mask shape 1702) are completelycross-linked during the post exposure bake and form a negative tone linepattern. Hybrid resist portions which are exposed with intermediateamounts of intensity (i.e., the areas under the edges of mask shape1702) become soluble in developer solution after the first exposure andare dissolved during the development step and form space 1808 in thehybrid resist.

Because portions of the hybrid resist were unexposed during the firstexposure, these regions remain photoactive and now comprise positivetone resist patterns. Thus, by blanket exposing the wafer these positivetone resist patterns are polymerized and can be washed away duringdevelopment. The blanket exposure is preferably an intermediateexposure, either by exposing at a low enough dose or for a short enoughtime to create an intermediate response to those areas of resist thatwere unexposed (i.e, the positive tone patterns) in the first exposurestep.

In the alternative, the positive tone portions can be removed by aselective etch using a solution of pure n-butyl acetate at roomtemperature or with a strong base, such as 0.35 Normal ("N") tetramethylammonium hydroxide ("TMAH").

Turning to FIGS. 21, 22, and 23, the wafer portion 1802 is illustratedafter a blanket exposure and development, where FIG. 22 is a crosssection of the wafer portion in FIG. 21 taken along lines 22--22, andFIG. 23 is a cross section of the wafer taken along lines 23--23.

The blanket exposure has caused the positive tone regions (i.e., thehybrid resist portion 1804 (of FIGS. 18, 19 and 20)) which wereunexposed (ie., blocked by mask shape 1702) to become soluble and washaway during development.

Preferred embodiments

The preferred embodiments of the present invention capitalize on theunique properties of x-ray lithography to optimize the use of hybridresist. In particular, the invention provides a method to produce hightolerance devices with x-ray pitch and a means to vary the space widthand to fine tune to account for process variations. Because of thenature of the hybrid resist discussed above, a space formed in a hybridresist corresponds to the transition from light to dark at the edge ofan aerial image. That is, the portion of the resist exposed to somepredetermined range of intermediate radiation intensity (less than fullintensity but more than zero intensity) will develop away to become aspace. Accordingly, the size of the space is a function of the sharpnessof the aerial image. Where an aerial image has a high contrast, thedistance over which the transition from full intensity to zero intensityoccurs may be relatively small. Also, where an aerial image has a lowcontrast, the distance over which the transition occurs will be largerand the developed space correspondingly larger. The present inventionmethod uses adjustments in the mask-wafer gap during x-ray exposure toform spaces in hybrid resist of a desirable dimension. Accordingly, apredetermined space width in the hybrid resist can be selectivelyprinted by varying the gap, allowing more versatile structures to beformed and adjustments to be made for process changes such as resistcomposition and ion implant levels.

Because a space in a hybrid resist will develop away when exposed to anintermediate level of actinic energy, the contrast of the aerial imageis the key to space width in x-ray lithography as well opticallithography. Image contrast determines the width of the region on aresist that receives intermediate exposure at the edge of an aerialimage projected onto the resist. However, contrast of the aerial imageis generally independent of exposure dose and mask image size in therange of interest. Since the area of intermediate exposure defines thespace width developed in a hybrid resist, the space width is alsoindependent of exposure dose and mask image size. Even so, the locationof a space is defined by the location of the edge of a mask image andthe width of the space may also be affected by resist formulation. Forthese reasons, abnormalities in a x-ray mask, such as variations in themask shape, will not cause abnormalities in the area of intermediateexposure nor the corresponding space width.

For example, if a 50 nm space is desired in a positive resist, then,generally speaking, the mask for the positive resist will have acorresponding 50 nm image. An abnormality can easily occur in the maskproduced by EBL such that the image is only 35 nm in one region of theimage. After fabricating a device using the 35 nm space, a device defectmay result since the device will be more narrow than the intended 50 nm.If using a hybrid resist, then the size of the space produced willremain constant and only its location will change slightly as defined bythe image edge in the region where the image is too narrow. A slightlocation change is less likely to cause a device defect as compared toimproper dimension. Thus, by using a hybrid resist in x-ray lithographyaccording to a preferred embodiment of the present invention, theabnormalities or defects in the mask typically do not transfer as devicedefects to the semiconductor. Accordingly, the advantages of smallerwavelength x-rays may be realized as smaller devices on a semiconductor.

While independent of mask size, space width in a hybrid resist is verydependent on aerial image contrast because the transition from zerointensity to full intensity defines the width of the hybrid resistregion that receives intermediate exposure. In x-ray lithography,because the mask image is proximity printed, aerial image contrast isdetermined by the gap distance between the resist layer and mask. Gapdistances of 10-50 μm are typical and, in conventional processes, thesmaller the gap the better since the image will generally have a highercontrast. Some caution must be exercised in varying gap width sincediffraction effects can yield counterproductive phenomena such aPoisson's bright spot. Nevertheless, according to a preferred embodimentof the present invention, when hybrid resists are combined with x-raylithography gap distance becomes a variable process parameter fordetermining space width. Depending on the resist formulation and otherfactors, the characteristics of an aerial image can be determined usingcomplex modeling to achieve, within certain bounds, a desired spacewidth in a hybrid resist.

EXAMPLE 5

This example illustrates the manner in which changing the type ofsolubility inhibitor on the resin can change the dissolution propertiesof a resist formulation to reduce resist loss in the dark areas of thex-ray pattern. A formulation was prepared in a manner similar to Example1, however, it included the following components:

PHS with about 25% of the phenol groups protected with methoxycyclohexene (MOCH), 89.7% of solids;

triphenyl sulfonium triflate, 2.5% of solids;

Powderlink, 7.8% of solids;

tetrabutylamnmonium hydroxide base, 0.1% of solids; and

sufficient PM acetate containing 350 ppm FC-430 surfactant as a solventto form a 13% solids solution.

The solution was filtered through a 0.2 μm filter then coated ontosilicon wafers primed with hexamethyldisilazane with a soft bake of 100°C. resulting in films of about 0.3 μm thick. The coated wafers were thenexposed with x-ray radiation with a wavelength of approximately 8 Å on aSuss stepper. The mask-wafer gap setting was 40 μm and the expose dosevalues ranged from 150 to 250 mj/cm². The expose mask consisted of agold or tantalum absorber about 0.5 μm thick, with a mask patterndimension of 0.35 to 0.8 μm. After expose, the wafers were baked at 95°C. for 90 seconds and then developed in 0.14 N TMAH for 150 seconds. Theresulting images provided a resist space width of about 60 nm.

FIGS. 24-26 show how a change in gap distance affects energy intensityas a function of position relative to the mask shape, which is relatedto contrast. FIG. 24 is a graph of the output from a conventional modelused to describe the actinic energy that passes through a x-ray maskimage. The model was used to estimate the energy intensity profile thatwould result from a mask of 150 nm equal lines and spaces, using a 420nm thick layer of gold as the mask material and providing a 25 μm gapdistance between the mask and resist. In FIG. 25, only the gap distancewas changed and equals 35 μm. Both FIGS. 24 and 25 show that, due todiffraction and other effects, the full intensity of x-rays directedthrough the mask diminishes near the edge of a space in the mask. Also,the two figures indicate that a change in the intensity profile occursas the gap distance changes. If the mask was juxtaposed against theresist layer with zero gap, then there would be very little, if any,diminishment of intensity at the space edge. However, as the mask-wafergap distance increases, intensity at the edges begins to diminish or, inthe other words, it lowers the aerial image contrast.

FIG. 26 provides a comparison of the intensity profile of FIG. 24superimposed on the profile of FIG. 25. As indicated by FIG. 26, for aresist layer exposed through a mask 35 μm from the resist, a widerregion will receive an intermediate level of energy than a resistexposed through a mask 25 μm from the resist. Noticeably, the 35 μmprofile drops off from full intensity further from the space edge thanthe 25 μm profile. Accordingly, a greater portion of the resist receivesa full intensity level from the 25 μm profile than from the 35 μmprofile.

The effect of the energy profile differences is shown in FIGS. 27 and28. FIG. 27 includes the same energy profile as FIG. 24, correspondingto a gap distance of 25 μm. If a hybrid resist is selected that willexhibit a positive tone response to energy levels between 0.5 and 1.2,then two 23 nm spaces 2700 will develop in the hybrid resist 2710. Thespace width shown in FIG. 27 is designated solely to provideillustration of the principle involved in the present invention and notby way of limitation or expectation. Whether the specified space widthwill actually result in a hybrid resist exposed through a 150 nm maskwill depend upon several other factors, for example, the resistformulation, the mask material, the wavelength produced from the x-raysource, and the developer solution(s).

FIG. 28 includes the same energy profile as FIG. 25, corresponding to agap distance of 35 μm. As described above, if a hybrid resist isselected that will exhibit a positive tone response to energy levelsbetween 0.5 and 1.2, then two spaces will develop in the hybrid resist.However, the spaces 2800 will be 30 nm in width due to the difference inthe energy profile at a 35 μm gap. That is, more of the resist isexposed to a level of energy within the intermediate range because theaerial image has a lower contrast.

The ability to vary the space width in a hybrid resist has at least twoprimary applications. First, it can be used to increase the versatilityof hybrid resist technology by forming spaces of varied width in a givenresist. Second, it can be used to fine tune the process of formingresist spaces by accounting for variations in resist chemistry, ionimplants, and other process conditions or steps.

As to the first primary application, for a given exposure wavelength inoptical lithography, the numerical aperture (NA) determines theresolution or sharpness of the aerial image projected onto the resistand, thus, determines the size of the resulting space on a hybridresist. The space width is generally unchanging as the exposure dose andthe reticle image size are changed and is, instead, largely dependent onthe chemical composition of the hybrid resist. This allows very preciseimage control for a set space width within each chip, across each wafer,and from one batch of product wafers to the next, provided the sameresist is used. However, the NA is set on the step-and-repeat tool forexposure of a given resist layer and then not changed because of thedifficulty and problems associated with changing NA. Since the NA isdifficult to change and other variables that might determine space widthare held constant, there is no means in optical lithography for varyingthe space width produced in a given hybrid resist layer deposited on awafer. Once the NA is selected, the space width is set and varied spacewidths cannot be easily produced in the same resist layer. Theversatility of hybrid resist technology may be increased in x-raylithography with the method according to a preferred embodiment forvarying the hybrid resist space width in a given resist layer.

As indicated above, hybrid resist portions that are unexposed (i.e., theregions blocked by a mask shape) remain photoactive and insoluble indeveloper. Accordingly, after a first exposure and development of spaceshaving a first width, the unexposed regions may be exposed through asecond mask at a second mask-wafer gap distance to produce spaces havinga second width. Alternatively, after a first exposure, but beforedevelopment of the first spaces, the unexposed regions could be exposedthrough a second mask at a second gap distance. Caution must beexercised in producing the spaces having a second width to prevent areaspreviously exposed at an intermediate level from becoming overexposedduring the second exposure. If an area intended to become a space isoverexposed, then the negative tone chemistry will be activated and itwill not later develop into a space. Accordingly, hybrid resist spacesof varying width are provided.

As to the second primary application, with typical resists, such as aconventional positive resist, a mechanism generally exists for finetuning the process to account for variations and still yield a productwithin tolerances. For example, if the chemical composition of a newbatch of positive resist material is slightly out of specification andwill result in space dimensions different from that required, thenadjustments can be made in exposure dose. For smaller than expectedspace dimensions in the conventional positive resist, the exposure doseis increased to more highly expose the image edges where diffractionpreviously limited the exposure. The increased exposure at the imageedges will accordingly increase the space dimensions to the needed size.Similarly, it may be that an ion implant step is improperly performed ina transfer device such that the performance of the associated transistorwill be affected. One conventional remedy used with typical resists isto increase the dimension of the space wherein the gate will be formedto yield a larger gate and to offset the ion implant error. Again, thespace dimension can be varied simply by changing the exposure dose ofthe conventional positive resist.

As indicated above, the space dimension in a hybrid resist isindependent of exposure so the previous methods for fine tuning toaccount for variations are not helpful with hybrid resists. The value ofhybrid resist technology may be improved in x-ray lithography with themethod according to a preferred embodiment for fine tuning the resistspaces by accounting for variations in resist chemistry, ion implants,and other process conditions or steps. For a given set of processconditions, an energy profile model can be used to predict the energyprofile that will exist and, accordingly, to predict how space widthwill change as mask-wafer gap distance is changed. These predictions canbe used to account for variance in process conditions. For example, ifthe chemical composition of a new batch of hybrid resist material isslightly out of specification and will result in space dimensionsdifferent from that required, then adjustments can be made in gapdistance. For larger than expected space dimensions in the resist (i.e.,not enough positive solubility inhibitor), the gap may be modified inaccordance with a predictive model to increase the contrast of theaerial image and more highly expose the image edges where diffractionpreviously limited the exposure. Modeling a predicted contrast isrequired since the change in contrast as a function of mask-wafer gap isnot a linear nor a monotonic response. However, an increased exposure atthe image edges will accordingly decrease the space dimensions.Similarly, it may be that an ion implant step is improperly performed ina transfer device such that the performance of the associated transistorwill be affected. Again, the space dimension can be varied by changingthe gap distance.

As described in FIG. 29, the two primary applications for the ability tovary space width in a hybrid resist can be combined into one novelmethod. A method 2900 according to a preferred embodiment of the presentinvention is a significant modification to a part of the much morecomplex conventional process of fabricating integrated circuit devices.The new method begins with a step 2910 of selecting a space widthdesired in the hybrid resist. Once a space width is selected, a step2920 is executed by determining a mask-wafer gap capable of yielding thedesired space width. Preferably, such determination is made by using anenergy profile model indicating an area of hybrid resist that willreceive an intermediate level of x-ray radiation depending on themask-wafer gap. FIGS. 24 to 28 offer examples of the type of output thatan energy profile model may yield. The model could also be furthermanipulated to automatically yield a gap distance in μm rather thangraphically plotting the profile and figuring the space width asexemplified by FIGS. 27 and 28. As such, the model may comprise acomputer program capable of calculating mask-wafer gap after input ofdesired space width, diffraction effects, and other process variablesthat influence space width.

Process variables are also involved in a step 2930 of checking whetheradjustments in mask-wafer gap are needed to account for variations inprocess variables. That is, resist formulation may change afterperforming the initial calculations wherein gap was determined orprocessing errors may occur, such as improper ion implantation,requiring modification of space width. If adjustment is needed, then themask-wafer gap can be modified as in step 2940 according to thepreferred embodiment to yield the desired space width. It may be, as inthe case of resist formulation changes, that the desired space width isthe width originally determined and gap modification will simplymaintain the desired width. Alternatively, it may be, as in the case ofimproper ion implantation, that the desired space width is differentfrom the originally determined width and gap modification will yield achange in space width.

Once a mask-wafer gap modification is made in step 2940 or determined tobe unnecessary in step 2930, then the hybrid resist is exposed through amask positioned at the established mask-wafer gap in step 2950. Asdiscussed earlier, the result of the exposure will yield a resist havingthree different regions of exposure. The portions that are unexposed(i.e., the inside region 1804 of FIG. 18 blocked by mask shape 1702)remain photoactive and insoluble in developer and form positive toneline patterns. Portions that are exposed with high intensity radiation(i.e., the outside region 1806 not blocked by mask shape 1702 ) willform a negative tone line pattern. Finally, other portions receiveintermediate amounts of intensity (i.e., the areas under the edges ofmask shape 1702) and are soluble in developer solution after the firstexposure.

After a first exposure, step 2960 allows the option of formingadditional structures of different width than the space width originallyselected in step 2910. Since a portion of the resist remains photoactiveafter step 2950, the photoactive portion can be subsequently exposedthrough a mask positioned at a different mask-wafer gap and, thus, yielda space having a correspondingly different space-width. Essentially,method 2900 allows for different space widths to be formed byre-executing steps 2910 to 2960 as long as photoactive portions of theresist remain. Care must be exercised in re-execution of the steps toavoid re-exposing areas that already received intermediate amounts ofintensity, otherwise, such areas might not develop into resist spaces.If no additional exposures are necessary as determined in step 2960,then processing may proceed to complete the integrated circuit device asin step 2960.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention. Accordingly, unless otherwise specified, any dimensions ofthe apparatus indicated in the drawings or herein are given as anexample of possible dimensions and not as a limitation. Similarly,unless otherwise specified, any sequence of steps of the methodindicated in the drawings or herein are given as an example of apossible sequence and not as a limitation.

We claim:
 1. A x-ray lithography method for forming spaces in a resist,the method comprising the steps of:a) selecting a first space width fora first hybrid resist space; b) determining a first mask-wafer gapcapable of yielding the first space width; c) depositing a layer of ahybrid resist on a semiconductor substrate; and d) exposing a firstregion of the hybrid resist to x-ray radiation through a mask positionedat the first mask-wafer gap.
 2. The method of claim 1, additionallycomprising the steps of:a) selecting a second space width for a secondhybrid resist space; b) determining a second mask-wafer gap capable ofyielding the second space width; and c) exposing a second region of thehybrid resist to x-ray radiation through a mask positioned at the secondmask-wafer gap.
 3. The method of claim 2, wherein the second region ofhybrid resist comprises a region that remains photoactive after exposingthe first region.
 4. The method of claim 1, wherein the step ofdetermining the first mask-wafer gap comprises using an energy profilemodel indicating an area of hybrid resist that will receive anintermediate level of x-ray radiation.
 5. A x-ray lithography method forforming spaces in a resist, the method comprising the steps of:a)identifying a process variation capable of necessitating a change inspace width upon formation of a hybrid resist space; b) determining achange in mask-wafer gap to a modified mask-wafer gap capable ofcounteracting the process variation; c) depositing a layer of a hybridresist on a semiconductor substrate; and d) exposing the hybrid resistto x-ray radiation through a mask positioned at the modified mask-wafergap.
 6. The method of claim 5, wherein the process variation comprises achange in hybrid resist composition or ion implant level.
 7. The methodof claim 5, wherein the step of determining a change in mask-wafer gapcomprises using an energy profile model indicating a change in an areaof hybrid resist that will receive an intermediate level of x-rayradiation.
 8. A method for forming a hybrid resist space using x-raylithography comprising the steps of:a) selecting a first space width fora first hybrid resist space; b) determining a first mask-wafer gapcapable of yielding the first space width; c) identifying a processvariation capable of necessitating a change in space width uponformation of the first hybrid resist space; d) determining a change inthe first mask-wafer gap to a modified first mask-wafer gap capable ofcounteracting the process variation; e) depositing a layer of a hybridresist on a semiconductor substrate; and f) exposing a first region ofthe hybrid resist to x-ray radiation through a mask positioned at themodified first mask-wafer gap.
 9. The method of claim 8, additionallycomprising the steps of:a) selecting a second space width for a secondhybrid resist space; b) determining a second mask-wafer gap capable ofyielding the second space width; and c) exposing a second region of thehybrid resist to x-ray radiation through a mask positioned at the secondmask-wafer gap.
 10. The method of claim 9, wherein the second region ofhybrid resist comprises a region that remains photoactive after exposingthe first region.
 11. The method of claim 8, wherein the processvariation comprises a change in hybrid resist composition or ion implantlevel.
 12. The method of claim 8, wherein the step of determining thefirst mask-wafer gap and a change in the first mask-wafer gap comprisesusing an energy profile model indicating an area of hybrid resist thatwill receive an intermediate level of x-ray radiation.
 13. A method forforming a hybrid resist space using x-ray lithography comprising thesteps of:a) selecting a first space width for a first hybrid resistspace; b) determining a first mask-wafer gap capable of yielding thefirst space width by using an energy profile model indicating an area ofhybrid resist that will receive an intermediate level of x-rayradiation; c) identifying a process variation capable of necessitating achange in space width upon formation of the first hybrid resist space;d) determining a change in the first mask-wafer gap to a modified firstmask-wafer gap capable of counteracting the process variation by usingan energy profile model indicating a change in the area of hybrid resistthat will receive an intermediate level of x-ray radiation; e)depositing a layer of a hybrid resist on a semiconductor substrate; andf) exposing a first region of the hybrid resist to x-ray radiationthrough a mask positioned at the modified first mask-wafer gap g)selecting a second space width for a second hybrid resist space; h)determining a second mask-wafer gap capable of yielding the second spacewidth; and i) exposing a second region of the hybrid resist to x-rayradiation through a mask positioned at the second mask-wafer gap,wherein the second region of hybrid resist comprises a region thatremains photoactive after exposing the first region.
 14. The method ofclaim 13, wherein the steps of determining the first mask-wafer gap anddetermining the change in the first mask-wafer gap occur simultaneously.15. A method for forming a hybrid resist space using x-ray lithographycomprising the steps of:a) selecting a first space width for a firsthybrid resist space; b) determining a first mask-wafer gap capable ofyielding the first space width; c) identifying a process variationcapable of necessitating a change in space width upon formation of thefirst hybrid resist space; d) determining a change in the firstmask-wafer gap to a modified first mask-wafer gap capable ofcounteracting the process variation; c) depositing a layer of a hybridresist on a semiconductor substrate; d) exposing the hybrid resist tox-ray radiation through a mask containing a plurality of shapes, whereinthe mask is positioned at the modified first mask-wafer gap such thatfirst portions of said hybrid resist are exposed to a first exposurelevel, second portions of said hybrid resist are exposed to an secondexposure level, and third portions of said hybrid resist are exposed toa third exposure level; and e) developing the hybrid resist such thatthe second portions of the hybrid resist are removed, forming the firsthybrid resist space.
 16. The method of claim 15, wherein the first levelof exposure leaves the first portion of hybrid resist photoactive,wherein the second level of exposure leaves the second portion of hybridresist soluble in developer, and wherein the third level of exposurecross links the third portion of said hybrid resist leaving it insolublein developer and no longer photoactive.
 17. The method of claim 16,additionally comprising the steps of:a) selecting a second space widthfor a second hybrid resist space; b) determining a second mask-wafer gapcapable of yielding the second space width; and c) exposing the hybridresist to x-ray radiation through a mask positioned at the secondmask-wafer gap such that additional portions of the hybrid resist attainan exposure level equivalent to the second portion.